It has been common practice for some time in the foundry industry to fabricate molds and cores, for use in casting metal parts, from commercial metal casting "plaster" which is a blend commonly comprising at least 50% gypsum plaster with the balance being primarily fibrous talc and some silica sand. In use, this plaster is mixed with a large amount of water, for example an equal or greater amount of water, to produce a highly fluid suspension which is capable of completely filling even relatively complex patterns in the master mold or pattern. Then this larger amount of water must be substantially completely eliminated, because any water which remains in the plaster can spoil a casting made therefrom, when it turns to steam upon contact with the molten metal, either by producing surface defects or by virtually exploding portions of the mold.
Drying of a plaster casting component by conventional methods is tedious and of unpredictable reliability in results, particularly if the component is complex or of substantial mass. One reason for these difficulties is that the gypsum component of the plaster normally retains a significant amount of water of crystalization, which cannot be eliminated without heating the entire component to a temperature greater than its calcining temperature of 270.degree. F. This is a very time-consuming operation with a conventional baking furnance, which can easily require as much as 30 hours at 300.degree. F, and even then, the probabilities are that a substantial proportion of a given plurality of components will crack or craze sufficiently to be unusable.
Attempts have been made to dry plaster casting components by exposure to microwave radiation in a microwave oven, on the premise that the known absorption capabilities of water for microwave radiation should make microwave heating an effective drying procedure for the plaster. Strangely, however, these attempts have not been successful, even when the mold or core is heated far beyond the normal 300.degree. temperature obtained in a conventional oven, for example even as high as 600.degree. F. While a mold or core dried in this manner appears to be completely dry, when it is then used for casting, sufficient additional water is given off by the plaster to spoil the majority of the castings. Additionally, heating to such high temperature ranges will usually cause cracks or crazing in a significant portion of the components which make them useless for casting purposes.